Control of rotation and accurate positioning of loads are important in handling loads by means of cranes. When using a single lifting cable the crane operator may control the load, or the centre of gravity of the load, in three dimensions, but not in a fourth dimension, the orientation of the load in the horizontal plane. To control and adjust the orientation of the load, manual adjustments or adjustments using auxiliary lines/wires has to be used. Manual handling/control of the orientation of loads may represent a significant hazard, especially if the load is heavy and/or have large dimensions along one or more axes. According to the Petroleum Safety Authority Norway, work involving cranes and the associated handling of the loads is the most common source of fatal accidents in the offshore industry. Thus, there is a need in the industry for a system to control the turning motion of a suspended load.
Additionally, it is often important to control the orientation of the load during the transfer of the load from the lift up position to the set down position, both to avoid physical hindrances and to ascertain that the orientation of the load is substantially correct to be set down to increase the efficiency.
The use of gyroscopic devices, i.e. devices based on spinning objects, for controlling the movement of different bodies, such as loads hanging in cranes or the like has been known for decades.
U.S. Pat. No. 1,645,079 relates to a stabilizer with two gyroscopic rotors and the use of the stabilizer for bomb-sights, cameras etc. in aircrafts. The device may be efficient in stabilizing a bomb-sight, camera or the like, but active relocation of the item to be stabilized from one orientation to another, is not described.
U.S. Pat. No. 5,871,249 describes a stable positioning system for a suspended payload, where a unit comprising a plurality of flywheels having axes of rotation being aligned with the three orthogonal axes. The system allows for stabilization of a suspended load but not for controlling the position and movement of a load.
The mentioned prior art is based on stabilization using the gyroscopic effect. The gyroscopic effect is well known in physics, and is based on the fact that if you apply a torque to a spinning object, the angular momentum will move in the direction of the torque. This means that if a torque τ is applied through the forces F in the vertical plane as shown in FIG. 1 the angular momentum L will move toward the torque and cause the spinning object to turn in the horizontal plane. Applied on a hanging load, this turning movement is a movement spinning around an axis of its own. FIGS. 2a) and b) illustrate two different arrangements of two spinning objects W, W′ on a load A having a center of gravitation at “X”, seen from above. Applying a force F, as illustrated in FIG. 1, to the spinning objects for a given duration, will create a torque in the direction indicated by the arrows. The torque will cause rotation of the load that is independent on the distance of the spinning objects from each other or from the center of gravity of the load. The torque is dependent on the spin and inertia of the spinning objects. The inertia of the spinning objects may be increased by increasing the rotation of the spinning objects, moving as much weigh as far as possible out from the axis of rotation, and increasing the weight of the spinning objects. Increasing the inertia of the spinning objects has its obvious limitations in the space available for this purpose, as have the total weight of the spinning objects, and the obtainable speed of rotation.
The torque creating effect on a load by tilting the spinning objects is limited as the torque will be oppositely directed when the spinning object is tilted more than 90 degrees from its starting position. The spinning objects then have to be repositioned relative to the body to be controlled, to be able to continue the required torque on the load. Clutches for disconnecting the load from the action of the spinning body, or reducing the speed of the spinning body for repositioning, have been proposed in the prior art.
U.S. Pat. No. 5,816,098 describes a lifting load attitude control system built on the principles above, the system comprising a flywheel suspended in a gyro frame to form a flywheel unit. Two or more flywheel units may be used together to increase the attitude control forces available. A clutch may be arranged between the flywheel unit and the load to be able to rotate the load independent of the flywheel unit. Disconnecting the load from the flywheel by the use of clutches for repositioning the flywheel has the negative effect of losing control of the load and its rotation for the time the load is disconnected.
Reducing the speed of the flywheel as in patent JP2797912 over the part of the rotation/tilt cycle where the torque from the flywheel is induced in the wrong direction of the intended one, will reduce the achieved rotational torque, and at best provide an alternating torque for load rotation.
An objective of the present invention is to provide a method and a system solving the non-solved problems according to the prior art solutions. More specifically, an objective of the present invention is to provide a method and system for controlling the rotation and thus the orientation of a hanging load, and to restart a rotation of the load towards a wanted orientation after a rotation of the load has been stopped or disturbed by external forces.